I met a guy at a campground who wants to put up a 1/4 wl verical on 40M but
doesnt have room for a good ground system. This is what he wants to try.

He has an old trampoline frame 16ft in dia that has about 120 springs on it
so he is going to run 120 wires from a center hub to the outer rail of the
frame.
each wire about 8 ft. Then run 5 or 6 wires spaced evenly around the frame
on out to the 1/4 wl mark. I was hoping there would be someone willing to
model something like this for him. All I have is the free version of EZNEC
and dont think it will handle the job.


Jimmie

On Fri, 10 Aug 2007 08:09:08 -0400, "Jimmie D"
wrote:

I met a guy at a campground who wants to put up a 1/4 wl verical on 40M but
doesnt have room for a good ground system. This is what he wants to try.

An interesting idea, but it is doomed by the springs. 120 springs
amount to 240 bad connections that will generate both noise and TVI.
Six radials is not bad for the vertical and I have to wonder if he can
do that many he can do a dozen.

Actually, the 120 wires planned on the trampoline will work very well
on the ground...

John Ferrell W8CCW
"Life is easier if you learn to
plow around the stumps"

"John Ferrell" wrote in message
...
On Fri, 10 Aug 2007 08:09:08 -0400, "Jimmie D"
wrote:

I met a guy at a campground who wants to put up a 1/4 wl verical on 40M
but
doesnt have room for a good ground system. This is what he wants to try.

An interesting idea, but it is doomed by the springs. 120 springs
amount to 240 bad connections that will generate both noise and TVI.
Six radials is not bad for the vertical and I have to wonder if he can
do that many he can do a dozen.

Actually, the 120 wires planned on the trampoline will work very well
on the ground...


We were concerned because they would be short, while we had room for the
trampoline frame there was only remove for a few full length radials.


Jimmie

On Sat, 11 Aug 2007 14:10:35 -0400, "Jimmie D"
wrote:
An interesting idea, but it is doomed by the springs. 120 springs
amount to 240 bad connections that will generate both noise and TVI.
Six radials is not bad for the vertical and I have to wonder if he can
do that many he can do a dozen.

Actually, the 120 wires planned on the trampoline will work very well
on the ground...


We were concerned because they would be short, while we had room for the
trampoline frame there was only remove for a few full length radials.

Hi Jimmie,

There is a simple fix to the Doom as it is practiced with spring mount
antennas on fenders and bumpers: short the spring with a wire slightly
longer than the maximum extension of the spring.

As for the actual wire inside, use Flexweave from the Wireman. It
will make storage infinitely easier.

The advantage of the frame and radial pattern will be found for a
shorter radiator (even if at the originally planned frequency). Hence
a tuner should accompany the package. If I have time this weekend, I
will run a model for you.

73's
Richard Clark, KB7QHC

On Sat, 11 Aug 2007 11:17:26 -0700, Richard Clark
wrote:

If I have time this weekend, I will run a model for you.

Hi Jimmie,

Time came sooner than I anticipated.

I presumed this tramp'o'tenna wouldn't be raised any higher than 3
feet (which I presumed to be roughly its normal stand configuration)
above ground.

With 120 8 foot radials and a 31 foot radiator, you do encounter a
mismatch with Z at 34 -j134 Ohms. Not bad actually, adding itenerant
extensions would bring in the match, but so would a tuner. I skipped
that exercise. The best launch characteristic stood at -1dB with a
total loss of 6dB. Raising the antenna did little to improve this,
and I doubt you would go 30 feet up to boost the output 1dB.

A shorter radiator of 16 feet with 8 8 foot top hat spokes slanting
down towards the radial rim (guys of part wire, part insulated line)
resulted in a Z at 17 -j8 Ohms. Adding more spokes did not improve
anything (I stumbled on this by accident). Performance was down at
-2dB and total loss at 7dB.

73's
Richard Clark, KB7QHC

Hello Jimmie:

Go ahead and spring for Roys Eznec. I think its up at version 5.0. If
you want to play your going to have to pay. It offers a the smith chart
for the antenna impedance which I like, for matching.

If a 1/4 wave 40 meter vertical is being used then it shouldn't be a big
deal to run the ground wires. I'd loose the springs tho.

I helped take down two towers of a SK, who
had his towers shut feed for 80 and 160, and used a massive ground
wires. This antenna system was all modeled on Eznec.

Jay in the Mojave

Jimmie D wrote:

I met a guy at a campground who wants to put up a 1/4 wl verical on
40M but doesnt have room for a good ground system. This is what he
wants to try.

He has an old trampoline frame 16ft in dia that has about 120 springs
on it so he is going to run 120 wires from a center hub to the outer
rail of the frame. each wire about 8 ft. Then run 5 or 6 wires spaced
evenly around the frame on out to the 1/4 wl mark. I was hoping there
would be someone willing to model something like this for him. All I
have is the free version of EZNEC and dont think it will handle the
job.


Jimmie

"Jay in the Mojave" wrote in message
...
Hello Jimmie:

Go ahead and spring for Roys Eznec. I think its up at version 5.0. If
you want to play your going to have to pay. It offers a the smith chart
for the antenna impedance which I like, for matching.

If a 1/4 wave 40 meter vertical is being used then it shouldn't be a big
deal to run the ground wires. I'd loose the springs tho.

I helped take down two towers of a SK, who
had his towers shut feed for 80 and 160, and used a massive ground
wires. This antenna system was all modeled on Eznec.

Jay in the Mojave
Im afraid if I did that I would kill more time on the PC than I do now.

Jimmie